When he just stay and do nothing, prepare for blast in straight lane, so stay slighty off his aim.
Dont tank any of his spells, they deal too much for low gear.
Save your W to clean off Fear if he cast it.
Dont try to engage him in fight if he is in avatar form, he will kill you no matter what.
Your F wont prevent him into getting into avatar form. Unless you use it before 50%, so he is silenced.
Good practice is to use T before he is 50%, about 60%, this way he will start casting spell and wont enter his avatar form. Then just stun him with E and F to easily kill him.